Six fire engines and around 40 firefighters were called to a fire at a leisure centre on Hyde Road in Hackney.
A member of public discovered the fire when they saw flames around the heater in the sauna on the first floor. They alerted a member of staff who investigated and operated the fire alarm system that automatically called the Brigade.
Around 80 people left the building before the arrival of the Brigade. There were no report of any injuries.
A small part of the first floor was damaged by fire.
The Brigade was called at 1717 and the fire was under control at 1900. Fire crews from Shoreditch, Islington, Whitechapel, Bethnal Green, Homerton, and Dowgate fire stations attended the scene.
The cause of the fire is under investigation.